The Extended JSON specification requires that the $-prefixed keys can appear in any order, e.g.

{b : {$binary : "AQID=", type : "02"}}
{b : {$type : "02", $binary : "AQID="}}

Currently JsonReader requires the keys to be in the specified order, so the second case above will be parsed as a normal document rather than as a BSON Binary.